The overall goal of this clinical research program is to provide infants with Down syndrome (DS) with the training and technology to independently explore their world to the same degree as their typically developing peers. Moving and mobility comprise up to 80% of a young child's waking hours. Such a high dose of sitting, standing and walking are a lofty gold standard for pediatric rehabilitation. Down syndrome (DS), which delays sitting, standing and walking by up to a year, significantly reduces infants' daily exploration wit serious cognitive, language, and social consequences. This proposal specifically tests the feasibility and effects of a novel 'mobile therapy environment' that provides powered mobility, progressive exercise and functional skills training in one assistive technology device. Our modified ride on cars specifically combine the fun and exploration of powered mobility (such as a Segway) with the ability to advance balance, strength and coordination through therapeutic exercises while infants practice sitting, standing and walking. Over the last year, we have developed a series of electrical and mechanical modifications that allow a single car to be progressively driven in sitting, then safely driven as triggered by standing (Figure 1 upper) then triggered by over ground walking (Figure 1 lower). This proposal will allow us to determine the feasibility and effect of ride on car training on sitting, standing and walking delays (Aim 1) and on infants' broader cognitive, language and social-emotional development (Aim 2). This proposal, inspired by our mobile robotics work and supported by preliminary data, is scientifically principled, innovative yet feasible within the funding period. The data and devices from this 2 yr project will support more formal work on the clinical and commercial potential of a novel category of assistive technology: the 'mobile therapy environment'. Figure 1 Modified ride on toy cars provide inexpensive effective mobility and socialization as well as strengthening, balance, coordination during driving while standing (Upper) and driving while walking (Lower).